Fix invalid UUID warnings for test_volume_*

Fix invalid UUID warnings for test_volume_*

Currently running 'tox -epy27 -- --regex test_volume' emits over 50
FutureWarnings from oslo_versioned_objects about invalid UUIDs [1].

This commit modifies relevant unit tests so that these warnings are
no longer emitted.

[1] http://docs.openstack.org/developer/oslo.versionedobjects/api/fields.html#oslo_versionedobjects.fields.UUIDField

Change-Id: Id165450ff4c1c7c6aac13a8916e2e72cfade199e